This invention relates to the field of switch mode power converters, and in particular fast dynamic response of power converters.
Switching converters are widely used to provide high efficiency and light weight power supply, however the transient output response to a fast load change is inherently limited by the output inductor that is present in most switching power converters. A typical prior art buck converter comprises a power stage with a plurality of switches, an inductor-capacitor filter and a feedback circuit. This feedback circuit monitors the converter output voltage and controls Pulse Width Modulation (PWM) of the switches. When there is a fast dynamic load change the response of the converter is limited by two parts of the converter, namely the feedback circuit and the power stage. The feedback circuit can be designed to be very fast by traditional linear approach or non-linear approach. However, the inherent response of the converter is limited by the output filter which is largely determined by the value of the output inductor.
Many researchers in the field have attempted to enable fast response through several different methods but all of the proposed solutions have limitations. Some researchers have attempted to speed up power converter dynamic response by using an inductor with small inductance value. This approach seemed to solve the problem because current delivery can rise much more rapidly through a small inductor. However, this approach brought on the problem of excessively high current ripple during normal operation, which introduces high root mean square current in the converter switches and passive components and this increases power loss. Other researchers have attempted to reduce losses by using parallel multiphase converters to share the current but this results in increased cost and complexity. Yet other researchers have attempted to address the problem by increasing the switching frequency. However, this introduces the problem of excessive switching losses in switches and magnetic losses in the inductor core. Additionally, high frequency operation requires high performance drive circuit which further escalates cost.
There is therefore an acute need for a method to provide fast response while maintaining a low loss level of the converter and at reduced cost for computer applications.
An invention U.S. Pat. No. 6,188,209 provides the basis for the present invention. The present invention further reduces complexity and provides alternatives to achieve fast transient response.
The present invention discloses different embodiments of an apparatus and a method with many salient features that provides fast transient response of switching power converters. The present invention dramatically increases the rate of change of current through an output inductor in a converter during transient while maintaining low current ripple at normal load. As it is not necessary to practice the present invention at high frequency, converter loss is kept to a minimum. However, applicant does not exclude the possibility of operating at high frequencies. The apparatus disclosed herein is operational with most power converter with an output inductor.
The basic approach of the disclosed method is to replace the inductor of a switching converter with one or more inductors with a higher inductance that operates during steady loading condition, and the capabilities to switch to one or more inductors with a lower inductance during a fast transient loading condition. This is accomplished by replacing the output inductor of a conventional buck switching converter with at least two series inductors, one of which has a small inductance, while the other has much higher inductance. The two terminals of the inductor with the higher inductance are programmed to be connected to a voltage source during transient condition. The voltage source can provide a rapid change of current in the output inductor with higher inductance while the inductor has been shorted to the voltage source. The connection to the voltage source reduces total equivalent series inductance of the two series inductor to the inductance of the inductor with small inductance, and enables high rate of change of current to the output load.
The voltage source used to short out the inductor can be any voltage in the converter, e.g. input voltage, output voltage or voltage drop of a switch or diode.
This invention produces low inductor ripple current in the steady state. In a specific embodinemt, during steady state operation, the equivalent series inductance of the series inductors is the summation of the two inductors. The inductor with high inductance is designed to be large enough to maintain very small ripple current to minimize the root mean square (RMS) current flowing through the switching elements and other components. The inductor with small inductance is designed to be small enough to provide necessary rate of change of current when the inductor with higher inductance is shorted out by a voltage source during transient condition. The transient conditions only exist for a short period of time and a converter spends most of its operating time in the steady state. Hence the converter will carry high ripple current only for a short duration of time and efficiency will not be seriously impaired. This invention is versatile and can be applied to most switching converters with output inductor.
In an alternative embodiment, two parallel inductors, one of which has a high inductance and the other having a much lower inductance, can also be employed similar to the description above, in providing large inductance during steady loading condition and switch to lower inductance during fast transient load change.
Accordingly, it is an object of the present invention to provide fast dynamic response to switching power converters.
It is another object of the present invention to maintain low output inductor ripple current.
It is another object of the present invention to improve converter dynamic response without operating at very high frequency.
It is another object of the present invention to maintain high converter efficiency.
It is another object of the present invention to use a simple control method.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and from the accompanying drawings.